add weights and conversion script for effecientnet

[divyashreepathihalli]

TODO:

• add weights conversion script here - https://github.com/keras-team/keras-hub/tree/master/tools/checkpoint_conversion
• Verify output with real inputs
• provide a working colab demo

Example PR - #1855.

[vulkomilev]

I will take this task

[pkgoogle]

Actually this is done: #1933